The present invention relates to circuit arrangements of the kind wherein a load is supplied via a triac with current from an alternating voltage supply source under the control of a triac trigger control means in combination with a sensing element. As a rule, the said alternating voltage supply source is formed by an alternating mains voltage source.
Usually the sensing element is sensitive to the effects produced by the load. For example, the sensing element may be a temperature dependent resistance located in proximity to a load in the form of a heating element, in which case variations of the sensing element may be utilised to control triggering of the triac to ensure that the heat produced by the load remains within certain temperature limits. Generally, in known circuit arrangements of the kind to which the invention relates, the sensing element is energized by a current from a direct current source so as to produce a voltage for influencing operation of said triac control means. For instance, circuit arrangements of the kind to which the invention relates are known in which the triac trigger control means form part of an integrated circuit assembly incorporating a direct current supply source which, in operation, is able to be energized by the alternating voltage source. Accordingly, the sensing element is readily able to be energized from the direct current supply source forming a part of the integrated circuit assembly. Known circuit arrangements employing direct current energisation of the said sensing element have the disadvantage that unless battery supplies are available as a direct current source it is necessary to provide a source of direct current by rectification and smoothing of alternating current derived from the alternating voltage supply source. The provision of battery supplies is expensive, likewise the provision of a rectification and smoothing circuit if a direct current source is energized from a mains supply. In instances where the triac trigger control means form a part of an integrated circuit assembly incorporating a direct current supply source energized by the said alternating voltage source, utilization of the incorporated direct current supply source has the limitation that typical incorporated direct current supply sources deliver voltages of less than 12 volts in order to suit the power supply needs of the triac control system. For the energisation of the sensing element from a source of direct current it is preferable for the source to have a terminal voltage significantly larger than 12 volts in order to utilise the available operating range of the sensing element. For example, the resistance of a thermistor employed as a sensing element for an electric cooking appliance may vary by three orders of magnitude over the anticipated cooking range.
With circuit arrangements of the kind to which the invention relates, in practice it may be necessary for the load and the sensing element to be located remotely from the remainder of the circuit arrangement with the consequence that connection leads between the remotely located sensing element and the remainder of the circuit arrangement are susceptible to the pick-up of interference which may impair satisfactory operation of the circuit arrangement when the sensing element is energized from a source of direct current, unless the interference is filtered. Moreover, circuit arrangements of the kind to which the invention relates frequently employ a temperature setting element in addition to a temperature sensing element, which temperature setting element is energized from the same source of direct current so that in the event of the temperature setting element being remotely located, the connection leads thereto are also susceptible to the pick-up of interference unless provision is made for filtering such interference.